1. Field of the Invention
The present invention relates to a solid state imaging device and a driving method thereof. More particularly, the present invention relates to a solid state imaging device using a MOS (Metal-Oxide-Semiconductor) image sensor of a threshold voltage modulation system, the solid state imaging device being used for a video camera, an electronic camera, an image input camera, a scanner, a facsimile or the like, and relates to a driving method thereof.
2. Description of the Prior Art
Since a semiconductor image sensor such as a CCD (Charge Coupled Device) image sensor and a MOS image sensor is excellent in mass productivity, the semiconductor image sensor has been applied to almost all types of image input devices accompanied with the progress in a pattern micro fabrication technology.
Particularly, in recent years, the applicability of the MOS image sensor has been recognized again because of the advantages thereof, i.e., smaller power consumption compared with that of the CCD image sensor, and the capability of making a sensor element and a peripheral circuit element by the same CMOS (Complimentary Metal-Oxide-Semiconductor) technology.
In consideration of such a social trend, the present inventor has improved the MOS image sensor, and filed a patent application regarding a sensor element having a carrier pocket (high-density buried layer) under a channel region (Japanese Paten Application No. Hei 10-186453) to obtain a patent thereof (Registered Number 2935492).
In the invention according to the patent (Registered Number 2935492), a photo diode 111 and an insulated gate field effect transistor for optical signal detection (hereinafter, occasionally referred to as a MOS transistor for optical signal detection, or simply as a MOS transistor) are formed as placed adjacently to each other. The MOS transistor is provided with a ring-shaped carrier pocket in a well region under a channel region. The carrier pocket has a higher acceptor density than that of a peripheral portion thereof and serves as a potential well storing holes as carriers. In a state where the well region including the carrier pocket is depleted, holes are generated in the well region of the photo diode by light irradiation, followed by transferring the optically generated holes and storing in the potential well of the carrier pocket. As a result, negative charges of acceptor ions in the carrier pocket are neutralized and a threshold voltage is changed.
Further, the above-described MOS image sensor has a circuit constitution shown in FIG. 8 of the patent (Registered Number 2935492). A series of operations of the MOS image sensor are passed through an initializing period, a storing period and a reading-out period by a control signal supplied from the CMOS circuit. During the initializing period, optically generated holes remaining in a hole pocket 25 are discharged to the substrate 11 through applying a positive voltage to each electrode. During the storing period, optically generated holes are generated by light irradiation and then stored in the carrier pocket 25. Then, during the reading-out period, an optical signal proportional to the storage amount of the optically generated holes is detected.
However, the control signal supplied from the CMOS circuit is directed to the trend of lowering a voltage thereof, and this trend is contrary to a request of more perfectly performing the initialization by applying a high voltage during the initializing period.
The first object of the present invention is to provide a solid state imaging device, which is capable of being maintaining a low-voltage operation of a CMOS circuit and performing more perfectly an initializing operation, and a driving method thereof.
Incidentally, with such a constitution, in the case where an acceptor density or a pattern shape in the carrier pocket 25 is not even, as shown in FIG. 10A, the potential does not become even over the entire region of the carrier pocket 25, and variation in height of the potential, that is, variation in depth of the potential well partially occurs occasionally.
In such a case, as shown in FIG. 10C, with regard to injection of holes having a potential at a low level, the holes are partialized at a place where the potential well is deeper, and a potential in the channel region is varied corresponding to this deviation. For this reason, there are problems that modulation of the threshold voltage does not become even in the channel region, element current distribution does not reflect storing distribution of the hole to lose linearity to a volume of stored holes, thus a so-called black batter occurs.
The second object of the present invention is to provide a driving method of a solid state imaging device, which is capable of modulating evenly a threshold voltage over the entire channel region of an insulated gate field effect transistor for light detection.
The present invention is related to the solid state imaging device. As a basic constitution thereof, as shown in FIG. 3, it is characterized in that the solid state imaging device of the present invention comprises a unit pixel 101 including a photo diode 111 and a MOS transistor 112 for optical signal detection placed adjacently to the photo diode 111, in which a gate electrode of the MOS transistor 112 is connected to a vertical scanning signal (VSCAN) driving scanning circuit 102, and a source region is connected to a voltage boost scanning circuit 108. Further, it is characterized in that the drain region is connected to the drain voltage (VDD) driving scanning circuit 103 in addition to the foregoing constitution.
Moreover, in unit pixel 101, as shown in FIG. 1 and FIG. 2A, the photo diode 111 and the MOS transistor 112 are formed in well regions 15a and 15b connected to each other, and the unit pixel 101 comprises a high-density buried layer (carrier pocket) 25 for storing optically generated charges in the well region 15b in a peripheral portion of the source region 16 of the MOS transistor 112.
In a driving method of the present invention, a voltage boost scanning circuit 108 is connected to the source region 16 of the MOS transistor 112 for optical signal detection, and in an initializing period, a boosted voltage is applied to the source region 16 from the voltage boost scanning circuit 108 in the state that the vertical scanning signal (VSCAN) driving scanning circuit 102 is isolated from the gate electrode 19, or in addition to the isolation of the circuit 102, the drain voltage (VDD) driving scanning circuit 103 is isolated from the drain region 17a. Thus, a boosted voltage higher than a power supply voltage of the VSCAN driving scanning circuit 102 is applied further to a gate electrode 19 through a capacitor between the source region 16 and the gate electrode 19 from the voltage boost scanning circuit 108, in addition to a gate voltage having applied thereto during the storing period. In other words, since a high voltage is applied to the source region 16 and the gate electrode 19, a sweeping operation of the stored charges from the carrier pocket 25 can be performed more perfectly.
As described above, as the solid state imaging device of the present invention comprises the boosted voltage scanning circuit 108, the first object of more perfectly performing the sweeping operation while maintaining to lower the outer supply voltage is attained.
Moreover, the present invention is related to a driving method of the solid state imaging device. As shown in FIG. 8, a voltage is applied to the gate electrode 19 of the insulated gate field effect transistor 112 for optical signal detection so that most of the optically generated charges stored in the carrier pocket 25 are discharged from the carrier pocket 25 during the initializing period and a specified amount thereof is made to remain in the carrier pocket 25.
In other words, the present invention, particularly as shown by a solid line of FIG. 9B, an appropriate voltage is applied to the source region 16 and the gate electrode 19 to make shallow the potential well of the carrier pocket 25, but to set the same in an appropriate depth, to discharge most of the optically generated charges and to let only a specified amount of the optically generated charges remain therein. The specified amount means the amount such that a potential distribution is flattened over the entire region of the carrier pocket 25. FIG. 10B, at this time, shows a model of a distribution of the optically generated charges over the entire region of the carrier pocket 25, and of the potential distribution thereover. In the case where the optically generated charges are stored further from such a state, the optically generated charges are stored without being deviated. Therefore, a modulation of the threshold voltage becomes even.
As described above, in a driving method of the solid state imaging device according to the present invention, such specified amount of the optically generated charges that the potential distribution is flattened over the entire region of the carrier pocket 25 is remained in the carrier pocket 25. Therefore, the second object that the modulation of the threshold voltage becomes even over the entire channel region of the MOS transistor in the storing period can be attained.
Note that, in the case where the well regions and the like are a conductive type reverse to the foregoing, that is, in the case where the high-density layer is an n-type, the high-density buried layer becomes an electron pocket (carrier pocket), and the optically generated electrons are stored. In this case, a negative large voltage is applied to the gate electrode to lower a potential of the surface of the well region 15b, and a potential well of the carrier pocket 25 is made shallow to discharge the optically generated electrons from the carrier pocket 25.